1. Field of the Invention The present invention relates to endoprostheses such as stents and stent-grafts, and particularly to endoprostheses that are suitable for use in transjugular intrahepatic portosystemic shunt (TIPS) applications.
2. Description of Related Art
It is known that diseased or damaged liver tissue may increase the resistance to hepatic perfusion resulting in excessive and often dangerous fluid pressure increases in the portal vascular circulation. This condition can lead to gastrointestinal variceal hemorrhage and pathological conditions such as ascites.
In order to decompress the portal circulation, a transjugular intrahepatic portosystemic shunt (TIPS) may be created through the liver tissue by connecting the portal vein to the inferior vena cava via the hepatic vein. This procedure first forms a fairly large puncture (for instance, using a 16 or 18 gauge needle) directly through the liver to allow direct flow between the portal vein and the hepatic vein. Next the puncture is lined with a stent (for example, an 8–12 mm stent) to form a shunt. The TIPS procedure has proven to be safe and effective at decompressing the portal system and in controlling acute variceal hemorrhage. A summary of TIPS procedures using unlined Palmaz balloon-expandable stents (available from Cordis Corp., Miami Lakes, Fla.) is provided in Zemel, et al., “Technical Advances in Transjugular Intrahepatic Portosystemic Shunts,” 12 RadioGraphics 615–622 (1992).
Unfortunately, for many patients conventional TIPS procedures may provide fairly short-lived improvements. Stenosis or occlusion may occur in up to half of all patients within 6 months following TIPS creation. Blockage of the shunt normally occurs due to pseudointimal formation or intimal hyperplastic response.
In the article by Nishimine, et al., “Improved Transjugular Intrahepatic Portosystemic Shunt Patency with PTFE Covered Stent-Grafts: Experimental Results in Swine,” 196 Radiology 341–347 (1995), the authors hypothesize that shunt patency might be improved if flowing blood could be separated from leaking bile, the exposed surface of liver parenchyma, and the injured hepatic vein. In that article the authors describe a device that comprises a thin-walled polytetrafluoroethylene (PTFE) graft (available from W. L. Gore & Associates, Inc., Flagstaff, Ariz.) having single-body Gianturco-Rosch Z-stents (available from Cook Inc., Bloomington, Ind.) mounted on each end for anchorage. Once mounted in place, the authors then deployed one or two WALLSTENT stents (available from Schneider Inc., Minneapolis, Minn.) within the PTFE graft to provide mid-shunt radial support. The authors reported that a PTFE-covered stent-graft provided improved TIPS patency verses uncovered stents in a porcine model. However, the authors also indicated that accurate stent-graft placement was important in order to maintain patency, since occlusion continued to occur when a stent-graft was misplaced in the shunt or if a portion of the intrahepatic tract between the portal vein and the hepatic vein was otherwise left unlined.
Nishimine et al. also reported that the two-step deployment of their stent-grafts (that is, deploying a graft with anchoring stents first and then separately deploying WALLSTENT stents within the graft as a second procedure) was “cumbersome and technically challenging.” Nevertheless, as the authors explained, this two-part procedure was necessary in order to allow placement of the stent and graft combination through a small 10-French (F) (3.3 mm) sheath.
Accordingly, a lined or otherwise covered stent-graft device would appear to be useful in helping to maintain patency in TIPS procedures. However, a number of problems are presented to someone attempting to provide an improved TIPS stent-graft.
First, as Nishimine et al. reported, small profile delivery of a stent-graft device is difficult to accomplish in a single step. Generally percutaneous delivery of a stent or stent-graft requires the device to be delivered at no more than 13-F (4.3 mm). While Nishimine et al. achieved delivery of a stent and graft through a 10-F sheath, this was accomplished through a cumbersome and challenging two-step procedure in which the separate devices were combined in-situ. Other authors have reported delivering stent and graft combined structures in one-step, but have required larger introductory profiles to do so, on the order of about 14-F to 16-F (4.7–5.3 mm): Haskal, et al, “PTFE Encapsulated Endovascular Stent-Graft for Transjugular Intrahepatic Shunts: Experimental Evaluation,” Radiology: 205 (1997); Behesti, et al. “Technical Considerations in Covering and Deploying a Wallstent Endoprosthesis for the Salvage of a Failing Transjugular Intrahepatic Portosystemic Shunt,” JVIR: 9 (1998).
Second, also as Nishimine et al. reported, accurate device sizing and placement may be critical in order to successfully avoid a biological response resulting in stenosis or occlusion. Premature device occlusion or shunt malfunction may be caused by a variety of conditions, including: failing to maintain proper flow into and out of the shunt device; failing to position the device to completely cover the intrahepatic tract between the portal and hepatic veins; or extending the device too far in the portal circulation or inferior vena cava.
Third, it appears important that the graft portion of the device inhibits seepage of bile and other thrombogenic or mitogenic substances into the blood system. Maintaining bile-exclusion while also seeking to use very thin graft materials in order to achieve small device delivery profiles presents a critical design challenge.
Fourth, the stent-graft also needs to have sufficient structural strength to resist distortion during use. Nishimine et al. report that protrusion of tissue through an uncovered stent structure may result in up to 40% of pseudointimal thickness. The present inventors believe that this tissue encroachment may be one of the reasons that uncovered stent structures do not perform very well. While any cover should limit the extent of tissue protruding through the stent structure, it is believed desirable to provide a cover with sufficient strength that it will resist distortion from the inward force of tissue protrusion. Thus, the choice of cover material, as well as its method of attachment to the underlying stent, present design challenges. The stent component also needs sufficient radial strength to prevent the stent-graft from significantly narrowing or collapsing under pressure from scar or fibrotic proliferation. Again, with respect to both the stent and the graft components, the amount of radial strength provided must be balanced against the desire for thin graft membranes and minimal stent size needed to create an overall device with small profile. Also, a desired attribute in the TIPS procedure is that the endoprosthesis is flexible in order to accommodate the tortuous intrahepatic pathway.